It's partly the horniness, but mostly the ego talking.
All men have a period of time (called the refractory period) immediately after orgasm in which their libidos are suppressed and it is not possible for them get it up or have another orgasm. During the refractory period, he couldn't get it up with a crane. Some men (especially young adults) are ready to go again in as little as a few minutes, but the average is probably more like 10-20 minutes. The length of the refractory period typically grows as a man ages, to the point where once might be all you're going to get with a senior citizen.
You two could enjoy lying naked in bed holding each other and gently caressing. That's plenty enjoyable in and of itself, and it will become readily self-evident when his body's ready for round 2. Have him rub your back and give you a massage while he's recovering. When you feel his hardness against your back, roll over and tell him that now that he's ready, there's something else you'd like him to rub.
There's no need to be upset or insulted if he needs a little time to get it up again - he really has no control over it. Bitching at him over it will only stress him and make it that much longer before he's back in action. I guarantee that if he COULD immediately get hard again, he very much would carry out his stated intention of making love to you all night long.
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